The present invention is directed to a simplex airblast fuel injection system and method for the atomization of fuel for ignition to drive a gas turbine and, more particularly, to such system and method which are capable of atomizing the fuel for ignition during both the startup and operation of the turbine.
A wide variety of fuel injection devices, systems and methods have been employed in the past for the atomization of fuel to support ignition and combustion for driving prime movers, such as gas turbines. These various devices, systems and methods each enjoy certain advantages, but they also suffer certain disadvantages. One common disadvantage is the difficulty of initiating fuel ignition during startup of the gas turbine. During startup the fuel must be presented in a sufficiently atomized condition to initiate and support ignition. However, the fuel and/or air pressures needed for such atomization are commonly unavailable during startup because the turbine, when it is simply being cranked at low rpm, is unable to produce sufficient fuel or air pressures which it otherwise is capable of providing in its running operating condition. Accordingly, various approaches have been taken in the past to achieve startup under these conditions. Some of these approaches ar generally discussed in Arthur H. Lefebvre, Atomization and Sprays, Hemisphere Publishing Corp., 1989, pages 136-142.
One approach has been to provide multiple circuit fuel systems and nozzles. One circuit and nozzle are of larger capacity for handling the normal fuel flow rates and pressures up to the maximum which are encountered over the range of operating conditions of the turbine. The other circuit and nozzle are specially configured and sized to handle fuel at the substantially lower flow rates and pressures existing during turbine startup to provide sufficient atomization of the fuel to initiate ignition and the starting of the turbine. These multiple circuit and nozzle fuel systems are more expensive, contain more parts, are less reliable and add to the total overall weight and complexity of the system.
In order to avoid the need for the aforementioned additional circuits and nozzles and their attendant disadvantages, air assist atomizers have found some usage in the field. In these air assist atomizers air or steam is employed to augment the atomization process at the low fuel injection pressure levels which exist at turbine startup. In the air assist atomizers provision is made to supply a high pressure, high velocity gas stream to assist the atomization of the relatively low velocity, low pressure liquid fuel stream issuing from the fuel nozzle during startup. However, the disadvantage of the air assist atomizer approach is the need for an external supply of high pressure air, such as by way of stored air from an air flask or the use of a separately powered air compressor. These requirements virtually eliminate the air assist atomizer for use in aircraft applications. Moreover, where the source of air is limited in volume, such as where air flasks are employed, the limited supply of air is used only during startup and is not utilized in the fuel atomization process during normal turbine operation.
Air blast atomizers have also been employed for the atomization of fuel to drive gas turbines. They are similar in some aspects to air assist atomizers, but unlike the latter they generally employ low pressure, low velocity air and they operate during the turbine operation, rather than at startup. Air blast atomizers have found widespread use in aircraft and marine, as well as industrial gas turbines because they enjoy several advantages over the pure simplex nozzle atomizers and the air assist atomizers. They can utilize lower fuel pump pressures, produce a finer spray and a more thorough mixing of the air and fuel, thereby resulting in a more efficient burn of the fuel, reduced soot formation, relatively lower flame radiation and a minimum of exhaust smoke.
Although the air blast atomizers enjoy a number of advantages in turbine operation, they typically suffer the disadvantage that they are incapable of startup without the use of ancillary equipment or procedures during conditions of low fuel and air pressure such as exists at the low turbine rpm startup conditions. One such provision which attempts to overcome this problem is the use of a "piloted" or "hybrid" atomizer in which a typical prefilming air blast atomizer is provided with an additional fuel circuit having a simplex nozzle designed simply to provide fuel which has been mechanically atomized without air assist at the low pressure startup conditions to initiate ignition. Once ignition has been initiated, another fuel atomizing system is utilized to provide the atomized fuel during the normal turbine operation and the role of the simplex nozzle becomes insignificant during normal operation. Thus, again in these "piloted" or "hybrid" atomizers, special fuel circuits and nozzles are needed which have little if any functional use or purpose during normal turbine operation.
In the present invention, a simplex airblast fuel injection system and method for the atomization of fuel for ignition to drive a gas turbine overcomes the several aforementioned disadvantages. In the system and method of the present invention the need for separate fuel circuits and nozzles which are only functional during turbine startup is avoided, as well as the additional expense, number of parts, reduced reliability and increased complexity which accompanies the use of such additional fuel circuits and nozzles. In the system and method of the present invention the need for additional sources of high pressure, high velocity air as in the air assist atomizers is also avoided. Thus, the system and method of the present invention are readily adapted to use in aircraft and marine turbines and have been found to be particularly well suited for use in small gas turbine engines which are battery started. In the system and method of the present invention the components thereof operatively function, both during the normal operation of the gas turbine, as well as during its startup, without the need to provide special components which are functional during only one of the last mentioned conditions and not the other.
It has been discovered in the present invention that a single simplex fuel nozzle and fuel supply system may be utilized during both startup and normal operation in conjunction with very low pressure, high volume air supplied by a turbine driven compressor during startup to achieve sufficient atomization to initiate ignition of the fuel. At the extremely low fuel pressures which exist at startup, a film is discharged from the nozzle orifice which by itself is incapable of atomization sufficient to initiate ignition. However, it has been discovered that if a swirling motion is imparted to the very low pressure, high volume air generated by the compressor at the low cranking speed rpm which exists under startup conditions, and this swirling air is directed toward the film so as to produce a vortex adjacent to the film as it issues from the nozzle orifice, a pressure differential can be established between the opposite sides of the film which is sufficient to explode the film to result in atomization sufficient to initiate ignition of the fuel. This discovery is surprising because it was not previously thought that the very low pressure, high volume air which exists at the low rpm cranking speeds was useful for this purpose. Hence recourse was previously made to the air assist principle with its high pressure, low volume air for fuel atomization during startup.
In one principal aspect of the present invention, a simplex air blast fuel injection system for the atomization of fuel for ignition to drive a gas turbine includes a simplex nozzle having an orifice and the nozzle is sized and constructed to produce a swirling stream of atomized fuel issuing from its orifice between maximum and minimum fuel pressures during the operation of the turbine. Fuel supply means supplies the fuel to the nozzle at pressures between the maximum and minimum fuel pressures during the turbine operation, and also supplies fuel to the nozzle during startup of the turbine, but at a pressure which is substantially lower than the minimum fuel pressure to form a fuel film issuing from the nozzle orifice which is insufficiently atomized to permit initiation of ignition. Air supply means supplies air at a low pressure and high volume to the fuel issuing from the nozzle orifice during both turbine startup and operation. Air directing means imparts a swirling motion to the low pressure, high volume air and directs the swirling air toward the fuel film as it issues from the orifice during startup of the turbine to explode and atomize the film sufficiently to permit initiation of ignition.
In another principal aspect of the system of the present invention, the aforementioned air supply means comprises a compressor powered by the turbine.
In still another principal aspect of the system of the present invention, the aforementioned fuel supply means includes a fuel pump powered by the turbine.
In still another principal aspect of the system of the present invention, the aforementioned air directing means also directs air at low pressure and high volume to the swirling stream of atomized fuel issuing from the orifice during turbine operation.
In still another principal aspect of the present invention, a method of atomizing fuel for ignition of the fuel to drive a gas turbine during startup includes supplying the fuel to a simplex nozzle at a substantially low pressure to form a film issuing from the orifice of the nozzle which is insufficiently atomized to initiate ignition; imparting a swirling motion to a stream of low pressure, high volume air; and directing the swirling stream of air toward the fuel film as it issues from the nozzle to explode and atomize the fuel in the film sufficiently to permit initiation of ignition during turbine startup.
In still another principal aspect of the present invention, a method of atomizing fuel for ignition to drive a gas turbine includes supplying fuel to a simplex nozzle from a fuel supply at a range of pressures between maximum and minimum during the operation of the turbine, and at a substantially lower pressure than the minimum fuel pressure during turbine startup; discharging the fuel from the nozzle as a swirling stream of atomized fuel during turbine operation, and during turbine startup as a film which is insufficiently atomized to initiate ignition; imparting a swirling motion to a stream of low pressure, high volume air; and directing the swirling stream of air toward the fuel film as it issues from the nozzle to explode and atomize the fuel film sufficiently to permit initiation of ignition during turbine startup.
In still another principal aspect of the method of the invention, the stream of low pressure, high volume air is also directed to the swirling stream of atomized fuel during turbine operation.
In still another principal aspect of the method of the invention, the fuel film is also swirling as it is discharged from the nozzle.
In still another principal aspect of the method of the invention, the direction of rotation of the swirling stream of air and fuel film are cocurrent.
These and other objects, features and advantages of the present invention will be more clearly understood upon consideration of the detailed description of the preferred embodiment of the invention which will be described to follow.